Probing the nature of the soft X-ray spectral flattening in high-z quasars

We propose to observe the quasar RXJ1028.60844 at z4.3 to investigate thenature of the intriguing soft Xray spectral flattening in highz quasars.Specifically, we will attempt to discriminate the hypotheses of absorptionor intrinsic spectral flattening, by detecting the Fe Kshell absorptionedge predicted by the former model. A positive detection of such an edgewill also allow us to determine the location and other physical parametersof the potential absorber.